Which loop to crochet under - Front Loop - Back Loop or Both Loops

If there is no indication to go through the front loop of a stitch or back loop, what I do is put my hook below both loops.  The reason is that I find it looks nicer and will be more secure than one loop.   For example,  when I was first learning to crochet, I was following instructions from a leaflet I had purchased.  It was for a shell afghan.  The instructions were telling me to put the hook through the back loop of the single crochet.  By the time I was done, I had big holes with each shell I had made. The shell is a heavy combination of crochet stitches, so it is bound to pull on the individual strand creating big holes.  Going under two strands will keep it more secure and evenly distribute the slack.  That taught me to go through both loops of the single crochet for that pattern and any other pattern that does not have a decorative purpose.  I also recognized that many of the patterns were advising to go through the back or front loop, when there is no decorative purpose. In the end, it all boils down to your personal preference and the combination of stitches that work best for you.